Amplifier system



1932- R. FELDTKELLER 1,872,264

AMPLIFIER SYSTEM Filed Aug. 24. 1929 INVENTOR RICHARD FELDTKELLER 7ATITORNEW Patented Aug. 16, 1932 its star RICHARD FELDTKELLER, OF BERLIN-CHARLOTTENBURG, GERMANY, ASSIGNOR .SIEIJIENS 8t EALSKE, AK'I'IENGESELLSCHAFT, OF SIEMENSSTADT, BERLIN,

GERMANY, A CORPORATION OF GERMANY AMPLIFIER svsrn ivr I Application filed August '24, 192$,Seria1 No. 388,153, and in Germany 3A1igu'st' '22f-1928.

An application for this invention has been filed in Germany, August 22, 1928.

(Df-a ll the coupling arrangements possible with amplifiers operating in series, the arrangement known in the prior art as resistance-capacity coupling has the reputation of producing multiple amplification which amplifies without distortionv a very broad band of frequencies. The same effect may, however, also be obtained with transformer coupled amplifier arrangements, and by means of the so-called longitudinal anti-distortion, in case the transformers have a sufficiently small straying effect. The transfonmer-coupled circuits are, also, well adapted for amplifiers which have an amplification curve, within a broad'frequency range, of definite dependence on'frequency.

Both coupling methods, however, have certain inherent faults. The resistance-capacity coupling does not produce apotential transformation from one tube tothe other, with the result that, due to the small amplification per tube, there are anore stages required. Furthermore, the coupling condenser -must have little leakage, otherwise a Wrong grid bias will'be produced. Condensers of this type are expensive.

While the transformer coupling allows an increased potential and, thereby, a larger amplification, yet the cores of the transformers are pre-magnetizedby the plate direct current in such a manner that This hardly possible to obtain sufficiently high inductances. Hencathe lower frequencies are not suflicient- 1y amplified. It is, likewise, not possible to overcome this difficulty by feeding the plate potential to the amplifier tube by meansof a special choke coil, and capacitatively coupling the intermediate transformer to the tube. In this case, thecore of'the plate choke coil is pre-magnetized with the result that its inductive apparent resistance becomes too small for the low frequencies. A loss in ainplification is, likewise, experienced in this case.

Besides, it is to be observed that, with the desirable use, of a common current source for the tubes arranged in series, the inductive apparent resistance in the plate circuit of each individual tube is not high enough for the low frequencies with the :result that there occurs a considerable drop ofthe iplateialternating current potential in the inner resistance of the common an'ode'current source. A

regenerative coupling is created, thereby lead ing to the-excitationof the amplifierofosciL lations with low frequency.

On the other band, due to the-pre-mag'netizing, the straying effect of the transformer,

or the choke coil leading to the plate, is also increased, creating the-danger'o-f a rohibitively strong inductive feed back. esides, with the increasing stray of the trans-former the frequency band becomes smaller, which may be transmitted, with'al-given arnplification, freeof distortion, or with a desireddistortion curve.

By means of the excessive enlaTgemehtof the coreand the windings :of'ithe *cho'ke coil, capacities of such size are obtainedth'atthere is rgenera-ted 'a stray resonance within the frequency range :to be transmitted which acts like a short-circuit in the out-put of the amplifier.

According to this invention the above enumerated disadvantages are avoided by means of a couplin'g'circuit arrangement which 'combines the features or characteristics of 'the discussed circuit arrangements. According to the invention the pla-tez'circuit is closed through a resistance to which is coupled "capacitatively the intermediate transformer. Resistances may be used for this purpose which possess the highest resistive properties known in the prior art. These are, for exam- 1ple,hard carbon resistances, practicallyaren inductive 'and'non capacitative.

In any case, care must be taken that the ohmicp art remains at least within the order of the size of the inner resistance of the amplifier also for the low frequencies. It is of special advantage toalso nouple capacitatively the secondary of "the intermediate transformer with the grid return resist'ance'in the grid circuit of the next following tube.

The novel feature which I believe to be characteristic of my inventionis set forth in particularity in the "appended claims, the invention itself,hovvever,-as tobc'th its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring to drawing which shows an en.- bodiment of the invention. the plate of the amplifier tube V1 is connected through a resistance 1 twith the positive pole AB of the plate source. On the other hand, a condenser C is inserted between the plate and the primary of the intermediate transformer U whose windings are collectively grounded, or connected to the heating battery H13. The second terminal of the secondary is connected through a condenser Q; to the grid of the amplifier tube V which grid is connected to the negative pole GB of the grid source through a resistance B A further stage may be connected in similar manner to the tube V To show the way this is accomplished, a further resistance R, is shown which connects the plate of this tube with the positive pole AB of the common plate source. Elements of the circuit arrangement not essential for the invention are not shown.

By means of the intermediate transformer U a potential transformation is secured and, therefore, an increase in the amplification. The plate current is kept away from the primary of the transformer U by means of the condenser C The possibility of a feed back through the common source is prevented by keeping theresistance R, in the plate circuit large; also, for very low frequencies, as compared with the inner resistance of the common plate source.

Although similar protecting measures, as provided for the plate circuit, are not necessary always for the grid circuit, the arrangement of the grid circuit is, as shown, suitable in many cases. Condenser C is the means of preventing the flowing of a grid direct current through the secondary of the intermediate transformer, while R furnishes the correct biasing potential in the grid. There may occur, for instance, during whistling of the tubes, considerable grid currents which, in case they flow through the transformer winding, result in disturbances. This is especially true in the case when the core of the transformer U consists of magnetic materials which are very sensitive towards a strong magnetizing action. For example, the high initial permeability of high-percentage iron alloys, such as iron silicide, nickel iron or the like is destroyed, under certain circumstances due to grid direct current in the secondary winding.

In many amplifier circuit arrangements there appearat the upper limit of the transformation range amplification peaks due to the stray resonance of the intermediate tran former. In such cases the grid resistance R may be dimensioned in such a manner as to damp these peaks.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications in the circuit arrangement, as well as in the appara tus employed, may be made without depart ing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. A combined resistance and transformer coupling means for a pair of triodes comprising a noninductiv=e and non-capacitative resistance in the anode circuit of one triode, a resistance in the grid circuit of the other triode, a coupling transformer connected between said resistances and a capacity connected between a high potential point of each resistance and a high potential point of said transformer. i

2. An amplifier circuit comprising a pair of triodes, a transformer coupling the output and input circuits of the triodes, a noninductive and non-capacitative resistance of high impedances in shunt with the transformer primary and connected in the said output circuit, a second resistance in shunt with the transformer secondary in the said input circuit, and a capacity connected in series between high potential points of each resistance and the transformer.

3. In combination, a pair of electron discharge tubes, combined resistance and inductive means for coupling the output circuit of one tube to the input circuit of the other tube comprising a transformer connected between the anode of one tube and the control electrode of the other tube, a source of potential for said anode, a high resistance of noninductive and non-capacitative characteristics in series with said anode and between the latter and its source of potential, a capacity connected to said anode and in series with said resistance and the primary of said transformer, and an electrical path, including a capacity and resistance in series, connected to the transformer secondary, said control electrode being connected to a pointon the path intermediate said capacity and resistance.

4. In combination, a pair of electron dis charge tubes, combined resistance and inductive means for coupling the output circuit of one tube to the input circuit of the other tube comprising a transformer connected between the anode of one tube and the control electrode of the other tube, a source of potential for the anode, a high resistance in series with said anode and between the latter and its source of potential, a capacity connected to said anode and in series with said resistance and the primary of said transformer, a source of biasing potential for said control electrode, and an electrical path, including a capacity and resistance in series, connected to the transformer secondary, said control electrode being connected to a point on the path between the capacity and resistance, said last mentioned resistance being connected between said control electrode and its source of biasing potential.

5. An amplifier comprising a pair of space discharge tubes, each of the tubes being provided with an input and output circuit, a current source for applying a positive potential to the anode of one of said tubes, a trans-- former coupling the input circuit of the second tube to the output circuit of the first tube, a high resistance connected in series between said anode and source, a condenser connected in series with each of said anodes,

resistance and the primary of said transformer, the condenser being disposed between high potential points of said resistance and primary, a source of biasing potential for the control electrode of the second tube, a grid leak resistor between the said control electrode and biasing source, and a condenser in series between the secondary of the transformer and said control electrode.

In testimony whereof I afiiX my signature.

RICHARD FELDTKELLER. 

